Television post production is the selection, processing and assembly of individual images and sounds to form a finished program of any length. Conventional post production is based on the process of selectively copying and processing scenes from a "source" tape to a record or destination tape. Generally, the control system allows transitions other than cuts including dissolves, wipes, digital effects, etc. The control system, in cooperation with a video switcher, also permits stills or moving keys to be placed over the source pictures in various ways. Because all picture elements must be presented simultaneously to switching and processing equipment and subsequently to the destination tape, the control system must be capable of operating a number of source tape machines which must be synchronized to a very high degree of accuracy. This requirement places severe limitations on the capability of a conventional editing system since transitions, other than cuts, require that scenes involved in the transitions be on separate reels of tape, mounted on separate machines.
Another drawback with conventional post production systems, is the quality degradations which occur with multiple generations. For instance, it is not uncommon to dissolve from one scene on a source tape to another scene on the same tape. Since both scenes must be presented simultaneously to the switching and processing equipment, a copy of one of the scenes must be made on a separate tape called a B-roll. Signal distortions introduced during the recording process, along with analog switching and processing devices, cause deterioration in the quality of the visual image being copied. Such generation losses through multiple generations of analog tape present a practical limit to the number of generations which may be used in conventional post production processes. Accordingly, conventional edit control systems have been designed to minimize the number of generations, and the industry standard edit decision list (EDL) is specifically geared to single pass operation.
The industry standard edit decision list provides a mechanism to trace each scene in the program back to the source from which it came. However, it does not provide an audit trail back to the original materials if the source is not itself an original. In other words, there is not automatic tracking through multiple generations. Further, the industry standard edit decision list has no mechanism for tracing non-video tape sources such as titles, live video from a camera, or sound recorded directly from a microphone. It does not accommodate any changes made to source material during editing, such as color correction or changes in picture size or position.